simulation of a superheterodyne receiver using pspice - School of ...

Fig. 23: Mixer Spectrum.
It can be observed from the spectrum in Fig. 35 that the desired signal is located at
455kHz. All the other spectral components will be filtered out as previously stated
with the tuned LC circuit. The desired signal will be fed to the IF stages, both of
which will be tuned to 455kHz and will have a bandwidth as close to 10kHz as
possible to ensure no desired information will be lost.
PART 5: DESIGN OF IF STAGE.
The output of the Mixer is amplified by two IF amplifier stages. Most of the receiver
sensitivity and selectivity is to be found in the IF stage. The two IF stages are tuned to
f IF
(= 455kHz). The highly amplified IF signal will then be applied to the Detector
where the original modulating signal is recovered. The IF amplifier is configured
using a BJT with a double tuned load. Consider the tuned LC circuit.
If Q>10 then we can approximate:
1
f
O
≈
2π
LC
1
∴C
≈
2
4π
f
2
O
= 455kHz
F
L
Choosing L=175uH then we get C=700pF from above. (Note: we also add in a small
resistance of 10 Ohms in series with the inductor).
Ref. [1] Appendix B
24